In order to produce lower cost solar cells to facilitate large scale electrical applications of solar electricity, it is important to provide lower cost substrate materials for making the solar cells. A known method for achieving this objective is to grow crystalline silicon using a continuous ribbon growth process as described in U.S. Pat. Nos. 4,661,200; 4,627,887; 4,689,109; and 4,594,229.
According to the continuous ribbon growth method, two high temperature material strings are introduced through holes in a crucible which contains a shallow layer of molten silicon. A crystalline silicon ribbon forms as the melt solidifies while being pulled vertically from the melt. The strings stabilize the edges of the growing ribbon. The molten silicon freezes into a solid ribbon just above the layer of molten silicon. To make this ribbon silicon process continuous, silicon is added to the melt as the crystalline silicon is formed to keep the amount of melt constant. Keeping the amount of the melt constant during the growth process is also important in order to achieve uniform and controllable growth of the crystalline silicon, and to keep the thermal environment of the cooling ribbon constant. Slight changes in the depth of the melt and consequent changes in the vertical position of the solid-liquid interface can significantly change this thermal environment. For example, it has been found that variations in the melt depth of more than about one millimeter can result in a markedly different thickness and introduce a residual stress state of the grown silicon ribbon. For all of these reasons, a constant melt level is an important element in insuring uniform, controlled growth of silicon ribbon. A method for continuously measuring the melt depth to provide feedback to a feed mechanism can be accomplished as described in the co-pending patent application titled, "Melt Depth Control for Semiconductor Materials Grown from a Melt," which is incorporated herein by reference. Once such a method is established, it then becomes important to be able to introduce a feed material at an accurate and predetermined rate. Since the solid silicon feed material must be melted, it is important that the introduction of solid silicon into the melt takes place with a minimum of thermal disruption in the immediate environment of the solid-liquid interface.
Controllable metering of feed material also has general application to crystal growth, where thermal upsets are not desirable, and a constant feed rate is needed. One example is in Czochralski growth of Si ingots, where it is desired to introduce additional feed material to the crucible during the growth. In this manner, a longer ingot of Si can be pulled from a single seeding.
Several methods of introducing a feed material into a crucible containing molten silicon are known. U.S. Pat. No. 4,036,595 describes a method wherein a separately heated crucible is used. European Patent No. 0 170 856 B1 describes a feeder with a moving belt which can be used for adding the feed silicon to a rotating cylindrical crucible with concentric dams to collect and melt the feed silicon in Czochralski crystal growth. U.S. Pat. No. 5,242,667 teaches a method which uses a silicon wiper blade on a horizontal, silicon rotating disc to control the feed rate from a storage hopper above the disc. All of these methods, however, have disadvantages. The first method requires a separately heated crucible. The second method requires a complex crucible arrangement that is only suitable for a cylindrical geometry, and may be difficult to implement for a feed material with a low angle of repose. The third method has limitations in controlling the feed rate for small feed rates such as would be found in silicon ribbon growth.
In order to controllably and continuously transport a silicon feed material for continuous silicon ribbon growth, it is important that the material have a morphology which lends itself to being readily and controllably transported. Silicon itself, if crushed, exhibits angular fracture and breakage along cleavage planes. This renders crushed silicon to be highly irregular in shape and thus difficult to controllably transport using a known method such as a vibratory feeder. Spherical silicon, on the other hand, can be produced by the fluidized bed decomposition of silane (SiH.sub.4) or in a shot tower. The former is a widely used method and presently is the source material for most silicon ribbon growth. This method produces spherical silicon with a size distribution from almost fine dust to about 2 mm in diameter.
U.S. Pat. No. 5,098,229 describes a silicon melt replenishment system which uses a pressurized fluid to blow silicon spheres up into a melt. One major disadvantage of this system is that it requires spherical silicon. Another major disadvantage is that the silicon spheres need to be in a fairly narrow particle size range to be effective. Spherical particles either too small or too large cannot be effectively used with this system. The result is that it is necessary to sieve the fluidized bed material to eliminate too large and too fine silicon spheres. The additional labor and handling of the sieving operation add to cost and increase the risk of impurity contamination.
Silicon ribbon grown for solar cell purposes is usually doped with a small amount of a dopant, typically Boron. Very small silicon pellets can be doped with Boron. Alternatively, very small amounts of Boron pellets can be added to the feed material consisting of silicon pellets and physically mixed with the silicon pellets before being transported into the molten silicon prior to growth. The mixing is done to promote a uniform distribution of Boron. However, in some transport systems such as a vibratory feeder, inhomogeneous mixing of Boron pellets or Boron containing silicon pellets can result. This can produce variations in the final bulk resistivity of the grown ribbon and this in turn can result in a less tightly controlled manufacturing process for solar cells.
A melt replenishment method that allows for continuous growth, which utilizes all sizes and morphologies of silicon, which allows for a simplified, low cost crucible design for silicon ribbon growth, and which produces a more homogeneous mixing of Boron, is thus very much needed and would represent a significant step towards low cost photovoltaics.